The human b- chemokines RANTES, MIP-1a and MIP-1b inhibit infection by primary isolates of HIV-1 but not by laboratory-adapted stains. We have now elucidated the mechanism by which these agents inhibit HIV-1 infection. Using a resonance energy transfer (RET)-based assay which measures HIV-1 envelope glycoprotein-mediated membrane fusion, we found that the b- chemokines inhibited fusion mediated by the macrophage-tropic primary isolate HIV-1(JR- FL), but not by the laboratory-adapted strain HIV-1(LAI). Furthermore, the chemokines had no effect on binding of HIV-1 gp120 to soluble CD4, demonstrating the b-chemokines inhibit HIV-1 infection at the first step of viral entry. These results imply that cellular b-chemokine receptors are accessory molecules for HIV-1 infection and mediate tropism of macrophage- tropic primary isolates. The goals of this Phase I SBIR proposal are to extend the preliminary results described above, using primary cells as fusion targets, including the analysis of infectious viruses expressing a variety of different HIV-1 envelope glycoproteins. Next, the precise mechanisms by which chemokines inhibit HIV-1 envelope-mediated membrane fusion will be examined. Finally, we will initiate the development of new drug candidates for HIV-1 infection, based on modified chemokines that can inhibit viral infection without initiating an inflammatory response.